1. Field of the Invention
The present invention relates generally to methods of measuring properties of a liquid material having more than one chemical component. More particularly, the present invention describes an apparatus and method for determining the amount of one or more components in a pulping liquor.
2. Description of the Prior Art
The kraft or sulphate process is the most widely used wood pulping process. The process is considered circular since the chemicals used to achieve the desired processing steps are themselves recovered in later steps and reused for further pulping of new raw material. The degree to which each step of the process occurs with the maximum efficiency greatly impacts the purity of the final white liquor as well as the cost of the overall process.
In the kraft process, wood chips are digested to dissolve the lignin that holds the wood fibers together thereby producing clean fibers for further processing into a myriad of paper-based products. The digestion of the wood chips occurs in an alkaline solution mainly consisting of NaOH (xe2x80x9chydroxidexe2x80x9d) and Na2S (xe2x80x9csulfidexe2x80x9d). As the process proceeds, the hydroxide becomes consumed and the sulfide slowly converts to hydroxide and maintains a residual throughout the cook. The resulting pulp fibers are washed and removed leaving a solution, called black liquor, containing the lignin dissolved from the wood chips and the residue hydroxide and sulfide. The black liquor is burned in a boiler leaving a smelt primarily consisting of sulfide and Na2CO3 (xe2x80x9ccarbonatexe2x80x9d). This smelt is dissolved in water or xe2x80x9cweak wash liquorxe2x80x9d to produce green liquor. The objective of the remaining steps of the process is to convert the carbonate of the green liquor to hydroxide so that the hydroxide can be recycled and reused in the pulping process.
The reaction for converting the carbonate to hydroxide is often referred to as the xe2x80x9ccausticizing processxe2x80x9d or the xe2x80x9ccausticizing reactionxe2x80x9d. The causticizing reaction, carried out in a xe2x80x9cslakerxe2x80x9d and a series of xe2x80x9ccausticizersxe2x80x9d, produces a material known as white liquor which ideally has a high degree of hydroxide and only a small amount of carbonate. An inefficient causticizing process results in relatively less hydroxide than ideal and more carbonate than ideal. The causticizing reaction is controlled by the amount of lime introduced to the slaker and the flow rate of green liquor into the slaker. To produce white liquor having the appropriate characteristics, lime must be input to the slaker at the appropriate rate. There are various known approaches for measuring characteristics of the green liquor and/or the white liquor and relating those measurements to the current state of the causticizing reaction. The objective of each of these known methods is to provide an appropriate signal for the control of lime introduction to the slaker. U.S. Pat. No. 4,236,960 issued to Hultman et al. on Dec. 2, 1980 describes one method for controlling the causticizing reaction. A sample stream of green liquor and a sample stream of white liquor are routed to a CO2 analyzer. A single CO2 analyzer is used to make sequential measurements of the liquors or two CO2 analyzers are used, one for each liquor. Each of the sample liquors is mixed with an acid solution so as to acidify the sample and convert carbonate to carbon dioxide gas. The amount of carbon dioxide gas is measured and the CO2 measurement is used as an indicator of the carbonate in the green liquor and in the white liquor. Various calculations are provided whereby the CO2 measurement for each liquor is related to the carbonate level in the liquor. The amount of lime introduced to the slaker is adjusted accordingly. The Hultman method measures only a sample of the liquor. The measurement is relatively complex in that it involves introducing an additional reaction to create a by-product, CO2, that can be measured. The measurement of CO2 is not directly related to the causticizing reaction and is therefore only an inferred measurement.
U.S. Pat. No. 4,536,253 issued to Bertelsen on Aug. 20, 1985 describes another method for controlling the causticizing reaction. Bertelsen teaches that the progress of the causticizing reaction can be measured by making a differential conductivity measurement. The conductivity of the green liquor is measured prior to the slaker and the conductivity of the white liquor is measured after the slaker. Equations are provided whereby the conductivity measurements are related to the progress of the causticizing reaction. The amount of lime introduced into the slaker is adjusted accordingly. White and green liquor are comprised of various components each of which has its own set of characteristics. A measurement of a single characteristic of the entire white or green liquor, as taught by Bertelsen, can result in errors. The major cause of these errors being concurrent changes in two or more components which mask the exact changes of each component separately. For example, green liquor has a small amount of hydroxide which contributes disproportionally to the conductivity measurement of the green liquor since the carbonate component of the green liquor has a relatively low conductivity. Thus, a relatively small variation in the amount of hydroxide in the green liquor results in a disproportionally significant change in the conductivity measurement of the green liquor. The opposite problem occurs on the other side of the causticizing reaction when measuring the conductivity of the white liquor. The Bertelsen method assumes that chemicals other than those of interest to the causticizing reaction are not present or do not vary in the measured liquors. This assumption is rarely true in practice and leads to errors in the Bertelsen measurement. The Bertelsen method also relies on a rigid mathematical formulation based on the conductivity measurements even though there are multiple components of each of the liquors, each of which can vary independently.
A further disadvantage of Bertelsen is that it makes use of a constant excitation voltage, and therefore measures the sum of all ions that react at or below that voltage. This constant voltage system typically also requires the use of a very well defined reference electrode, normally a liquid junction. These precise electrodes typically have a very short lifespan. Constant voltages systems can also be easily disturbed as the analyses temperature or composition changes, because these factors affect the ideal measurement potential.
While at fist glance some types of measurement systems outside the paper industry might seem applicable to improve on the above types of measurements, these systems are typically designed as low current systems, and are incompatible with the highly conductive liquors of the paper industry. Ion specific electrodes, such as Alpha model ISE-8750 sold by Omega Engineering, Inc., can measure Carbonate, but only up to 440 PPM (0.4 grams/liter) Additionally, these electrodes become fouled or poisoned by the sulfur in the liquor. The same is true for pH electrodes, which can sense the hydroxide imbalance in the solution. The pH electrodes become easily poisoned, and are not accurate at the high hydroxide concentrations of white liquor.
Thus, there exists a need for a continuous measurement of active chemicals in the causticizing and re-causticizing processes. There exists a further need for a continuous measurement which provides significant improvement over one that only occurs at periodic intervals, as is the case with sampling or off-line systems. There exists a need for measuring characteristics of individual components of the green liquor and the white liquor during the causticizing and re-causticizing reactions to prevent masking of a change in one liquor component by another. There exists a need for accomplishing the above in the face of changing process parameters, such as pH, temperature, unknown impurities and other factors which influence concentration measurements. There exists a further need to accomplish the above using durable sensor components to reduce process and/or measurement system down time and maintenance costs.
The Applicant has solved these and other problems by providing a method and apparatus for measuring liquor components, such as those in the green liquor and white liquor which result during a causticizing process. These measurements can be used to achieve more effective control of the causticizing or other (i.e. digestive) reaction for the production of white liquor or other desired output products (e.g., pulp consistency, degree of digestion, etc.). The Applicant""s invention further allows measurement of each relevant component of each of the liquors to occur concurrently. These results are achieved in a system that continuously measures the stream of liquor as opposed to sampling the liquor stream, and is effective even in the face of changing process parameters, such as pH, temperature, unknown impurities and other factors which influence concentration measurements. Further, the Applicant""s invention allows durable sensor components to be used which would otherwise be non-ideal due to their difficulty in maintaining a stable voltage.
The Applicant""s invention comprises the following steps. At a first electrode, a varying voltage is supplied in a voltage range including the half-wave potential of each component of a liquor to be measured, and accounting for variations in the half-wave potential caused by changing process parameters. At a second electrode, which can be roughly ⅓ to xc2xc the size of the first electrode, the derivative of current intensity is monitored near the known half-wave potential for the various liquor components. Using look-up tables, iterative curve-fitting means or other methods, the derivative of current intensity and selected other process condition data is used to determine concentrations of the various liquor components.
The electrodes may be composed of platinum or a similar inert material with little worry about conductivity variations of the electrode material. Gold, carbon, graphite, iridium, and palladium are all largely inert and my serve as viable options as well. Other metals such as Stainless steel and titanium are possible within limited voltage ranges.